1. Field of the Invention
This invention relates to techniques for performing bistatic radar functions when a transmitter platform is in motion.
2. Brief Description of the Related Art
Radar systems may be monostatic, bistatic or multistatic systems. Monostatic radar systems consist of a transmitter and receiver that are collocated on the same platform. A bistatic radar system performs radar functions but does not require the transmitter and receiver to be collocated. A multistatic radar system has multiple receivers that are separated from one transmitter or one receiver that is separated from multiple transmitters or multiple receivers that are separated from multiple transmitters. Such transmitter and receiver platforms may be stationary (ground-based) or may be in motion.
A transmitted radar signal is scattered by all material objects. Knowledge of the transmitted signal is desirable at the receiver if information is to be extracted from the target path signal. The strength of the scattered signal is determined by the direction and orientation of the source relative to the radar antenna beam directions, by the distances from the radar to the source, by electromagnetic properties of the source and the electromagnetic properties of the transmitter and receiver hardware. A time or phase reference is desired if the total scattered path length is to be determined. The transmitted frequency also is desired to determine the Doppler frequency shift. In a bistatic or multistatic system, the time reference also may be obtained from the direct path signal provided the distance between the transmitter and the receiver is known. The frequency reference may be obtained from a direct path signal received from the transmitter provided the transmitter and receiver velocities are known.
The material objects that scatter the transmitted signal define a multipath environment. The multipath environment will, in general, include objects that are of interest to the radar operator as well as objects that are not of interest. Those objects of interest in the multipath environment are called targets. Those objects that are not of interest are called clutter. Ground clutter refers to the multipath sources on the earth's surface and includes both natural vegetation and man-made structures. Ground clutter is often modeled as stationary, but does include motion due to the rotation of the earth and may have internal motion, such as due to wind. Ground clutter may also include surface vehicles, animals or people and may be non-stationary due to motion from one point to another during the period that the radar is operating.
Decisions about the presence of a target of interest are called detection decisions. Detection decisions are based on the output products of algorithms that process the radar signal.
Known radar systems may transmit a signal beam in a specific direction to search for targets. Once a target has been detected, a beam or multiple beams may be directed to follow the target. The receiver may receive scattered signals reflected off the target. By knowing the transmitter beam parameters, the receiver may perform operations to determine the target parameters, as discussed above.
The performance of a radar system is characterized by its capability to reliably detect targets of interest while maintaining a low false alarm rate and by its capability to derive accurate estimates of target position and motion. For some radar systems, performance may be further characterized by the capability to classify or identify the target of interest.
Radar measurements are derived from the received radar signal. Measurements commonly include signal strength, delay, Doppler and angle of arrival. Delay corresponds to the propagation time from transmitter to scatterer and back to receiver. Doppler corresponds to the shift in frequency of the scattered signal relative to the transmitted signal. The angle of arrival refers to the direction from the radar antenna to the scatterer relative to the boresight direction of the antenna.
Measurements may be computed prior to, simultaneously with or after detection decisions. Algorithms may be applied to measurements to derive estimates of the location and motion of a scattering source.
An ideal radar scenario is one for which the roll, yaw and pitch of the radar platform can be ignored, one for which radar hardware imperfections such as array mis-alignment can be ignored and one for which clutter motion, clutter inhomegeneity and jammers can be ignored.
The cone angle is the angle between the radar platform heading and a vector from the platform to a clutter sample. For linear side-looking arrays, the azimuth angle is defined as π/2−θcone 
The clutter locus describes the simultaneous measurement of Doppler and the sine of the cone angle for clutter samples distributed over the radar field of view (FOV). The clutter locus depends on the radar system and the clutter environment. For a selected ideal radar system, the clutter locus associated obtained for a selected radar scenario is called the Characteristic Clutter Locus.
A display of a Characteristic Clutter Locus for an ideal monostatic radar is shown in FIG. 1. The space-time aperture represents the simultaneous measurement of Doppler and the sine of azimuth angle of arrival. FIG. 1 shows the distribution of clutter measurements in the space-time aperture for typical airborne radar. The clutter includes sources that are spread over multiple range cells. The Doppler of a clutter sample depends directly on the azimuth of the clutter sample relative to the platform.
The Characteristic Clutter Locus for an ideal monostatic radar is linear. The slope of the linear clutter locus depends on the magnitude of the platform velocity and the orientation (θ0) of the array relative to the platform motion.
The slope of the Characteristic Clutter Locus for an ideal monostatic radar is delay independent. When ambiguous sidelobe returns are included, the Characteristic Clutter Locus will be a set of equally spaced lines each with the same slope.
The performance of a radar system can be compared to a notional system in which clutter is not present. The difference between the received signal in an actual radar system and the notional radar system is called the clutter signal. The difference in performance between a radar system and the clutter-free radar system in which clutter is not present is called the degradation in performance due to clutter. The clutter signal is treated as one type of interference that degrades radar system performance.
The radar system performance may be also be degraded by a component of the transmitted signal which propagates from the transmitter directly into the receiver system without scattering by the multipath environment. This is a second type of interference and called direct path interference.
A third type of interference is due to transmitters that operate either intentionally or unintentionally at or near the radar center frequency. Transmitters that operate intentionally at or near the radar center frequency are called jammers. Transmitters that operate unintentionally are called co-channel transmitters. The interference signal due to jammers and co-channel transmitters may also include the effect of scattering by the multipath environment.
For monostatic radars, pulsed waveforms have been traditionally used by radars to mitigate direct path interference.
For monostatic radars, adaptive antennas have been developed to suppress the direct path component of jammer and co-channel interference sources. Adaptive antenna technology requires a radar antenna that has multiple, independent channels. Independent spatial channels may be derived from the output of distinct elements in a phased array, from the output of distinct sub-arrays that are created as a combination of feeds or elements or from separate antenna beams formed as a combination of weighted data at the output of antenna elements or feeds. The technology to form sub-arrays or multiple beams can be embedded in the design of the antenna hardware or implemented as a module or modules in the digital signal processor. It is adaptive in the sense that processing parameters depend on the interference signal. Adaptive antenna technology has been developed for ground-based radars and for radars on moving platforms.
For monostatic ground-based radars, moving target indication (MTI) and Doppler radar were developed to suppress the stationary ground clutter and improve the detectability of moving targets. For such a ground-based radar, all clutter has zero-Doppler and is concentrated in a single Doppler measurement cell. A filter designed to cancel zero-Doppler measurements has the effect of suppressing background clutter. As long as the target is not flying in a direction orthogonal to the radar's look direction (in which case it is as zero-Doppler and is filtered out along with the background clutter), the target signal passes through the filter without significant attenuation or distortion and target detection is no longer strongly degraded by the presence of background clutter.
In monostatic airborne and space borne radar systems, ground clutter is a greater challenge primarily because it is spread throughout a large region of the delay and Doppler measurement space. The spread of ground clutter is, in general, a direct result of the radar's platform motion. The ground clutter generates an interference signal that is distributed throughout the measurement space and masks targets that might otherwise be detectable.
Displaced Phase Center Aperture (DPCA) processing and Space-time adaptive processing (STAP) were invented as a technique to suppress stationary ground clutter when monostatic radars are deployed on moving platforms for the purpose of detecting moving targets.
DPCA sensor and processing parameters do not depend on the clutter environment and in this sense are non-adaptive or deterministic. DPCA is applied to receiver data before detection processing and measurement estimation.
At the foundation of monostatic clutter suppression techniques such as STAP is the observation that clutter can be discriminated from target signals, even when they occupy the same range and Doppler measurement cell. The Doppler of a clutter cell depends linearly on the cosine of cone angle of the clutter relative to the platform heading. Conceptually, this linear relationship defines, for each Doppler cell, a unique cone angle associated with the clutter source. A moving target for which the velocity component in direction of the radar motion is non-zero cannot have identical Doppler and cone angle measurements as the clutter. The difference between target and clutter cone angle measurement increases as a function of the target velocity component in the direction of the radar platform heading. The placement of a null in the antenna pattern in the direction of the clutter source will effectively eliminate clutter returns for that Doppler cell. Moving target returns may also be attenuated but this attenuation will be much less than the attenuation of clutter for those targets with sufficiently large velocity component in the direction of the radar platform heading. Thus, the detectability of moving targets is improved. As the speed of a target is reduced, the characteristics of the target signal will become similar to the characteristics of the clutter signal and will be suppressed along with clutter.
The quantity, Minimum Detectable Velocity (MDV), characterizes the capability of DPCA and STAP algorithms to detect slow-moving targets. Signal to Interference-Plus-Noise Ratio (SINR) compares the strength of target signals to the combined signal consisting of interference or clutter and noise. SINR Loss describes the characterizes the performance of DPCA and STAP algorithms to a interference-free environment.
STAP improves performance in the presence of both clutter and interference due to jammers or co-channel transmitters. STAP can further compensate for mismatch in the RF characteristics between antenna and receiver subsystems for the independent radar phase centers.
The application of STAP algorithms to radar data requires a radar antenna, similar to that described for adaptive antennas, with multiple independent channels or outputs.
Coherent digital signal processing is commonly used to filter each channel of received data, prior to STAP processing, into a set of multiple data streams that are naturally associated with multipath sources at approximately, the same delay. Each specific delay value and the variation in the delay value of filtered clutter samples is termed a delay bin. The variation in the delay value depends on the signal bandwidth and is commonly referred to as delay resolution or equivalently the extent of the delay bin. The filtering of each channel of receiver data into delay bins is commonly called delay processing or matched filter processing. The pulse repetition interval (PRI) defines the segment of transmitted signal that is used to compute the matched filter output. The Pulse Repetition Frequency (PRF) is defined as the inverse of the PRI.
For waveforms designed as a repeating pulse train, the PRI corresponds to the interval between two successive pulses. For non-recurring waveforms, the PRI is not naturally defined by the signal structure and can be specified as an independent signal or processing parameter. The extent of PRI in this case may be defined, for example, to ensure that certain MF performance measures are met. Performance measures might include expected signal-to-noise ratio (SNR) or de-correlation due to expected motion of scatterers during the PRI.
Coherent digital signal processing can also be used to filter each channel into a set of multiple data streams that are naturally associated with multipath sources at approximately the same delay and at approximately the same Doppler shift. Each specific delay and Doppler value and the variation in the delay and Doppler value of filtered clutter samples is termed a delay-Doppler or measurement bin. The variation in the Doppler value depends on the duration of the coherent integration interval and is commonly referred to as Doppler resolution or equivalently the extent of the Doppler bin. The filtering of each channel of receiver data into delay-Doppler bins is commonly called delay-Doppler processing, ambiguity surface processing and complex ambiguity function (CAF) processing. The coherent processing interval (CPI) defines the segment of transmitted signal that is used to compute the CAF output.
Degrees of Freedom (DOF's) correspond to data values at the output of the radar signal processor. A three-dimensional index is commonly used to parameterize DOF's. The three-dimensional index identifies a unique receiver element called the spatial DOF, a unique PRI called the slow-time DOF and a unique delay bin called the fast-time DOF.
The number of spatial DOF's is represented by K, the number of fast-time DOF's by M and the number of slow-time DOF's by N. The total number of independent DOF's is the product of temporal and spatial DOF's and is equal to K·M·N.
Statistical properties of the interference signal are described by the covariance, R, between the spatial and temporal DOF's. The eigenvalues and eigenvectors of the covariance provide an equivalent representation or basis for the statistical properties of the interference signal. The number of eigenvalues/eigenvectors is equal to K·M·N, the number of independent DOF's. The order or rank of a covariance matrix is equal to the number of eigenvalues/eigenvectors.
In some cases, it may be possible to describe the statistical characteristics of clutter and other sources of interference by a lower dimensional set of basis vectors. The rank of the clutter and/or interference is defined as the number of basis vectors required for this representation. The rank of clutter or interference data may be less than the rank of the full covariance matrix.
For a monostatic radar scenario, the clutter rank may be significantly less than the rank of the full covariance matrix. For an ideal monostatic radar scenario, fast-time DOF's are not required for clutter suppression and M is typically set equal to 1. The clutter rank is an estimate of the number of significant eigenvalues. For an ideal monostatic radar scenario, the rank of the interference is approximately K+(N−1)β where β is a factor of 2 times the ratio of distance traveled by the radar platform during the time between radar pulses and the separation between spatial DOF's. A measure of the rank reduction is the ratio of K+(N−1)β to the product K·N.
The reduced rank of clutter is a direct consequence of the linearity of the clutter locus. This is commonly referred to as “Brennan's rule”.
The desired response for a STAP processor is defined by a space-time steering vector, {overscore (v)}.
The optimum STAP processor, {overscore (w)}, depends on the covariance and the selected steering vector: {overscore (w)}=R−1{overscore (v)}. The computational complexity is typically dominated by the need to determine the inverse of the covariance.
In practice, an estimate of the covariance for a selected delay bin is derived from data in neighboring bins. This data is referred to as training data. The clutter rank for training data in an ideal monostatic scenario that is the same as the clutter rank for the selected delay bin. In addition, for an ideal monostatic scenario, the statistical characteristics of covariance estimate based on training data are similar to that of the ideal covariance and when the amount of training data is approximately twice the order of the covariance, the achieved performance is roughly within 3 dB of that for the optimum STAP processor.
Because the both the computational complexity and the size of training data increase with the order of the covariance, techniques to reduce the rank or dimensionality of data at the input of the STAP processor are commonly employed.
One class of techniques uses non-adaptive or deterministic processing and is referred to as subspace projection. Sub-space projection techniques include Post-Doppler STAP in which slow-time DOF's are defined at the output of CAF processing. Staggered PRF, as an example of Post-Doppler STAP techniques. For Staggered PRF, the slow-time DOF's are defined by a sequence of over-lapping CPI's. Adjacent cell Post-Doppler STAP is another example of Post-Doppler STAP techniques where slow-time DOF's are derived from data in neighboring Doppler bins.
Sub-space projection techniques also include Beamspace STAP in which spatial DOF's are reduced by application of a set of distinct, fixed aperture weights to the spatial DOF's and the summation of the weighted data. The fixed aperture weights may be employed to form a set of fixed, overlapping radar beams where each radar beam has a unique beam center. The fixed aperture weights may also be employed to define overlapping sub-arrays where the phase centers of the set of sub-arrays are spatially separated. The formation of independent and distinct beams or sub-arrays for Beamspace STAP may be embedded in the design of the antenna hardware or implemented as a module or modules in the digital signal processor.
Post-Doppler and Beamspace STAP techniques can be employed separately or in combination as a STAP pre-processor such as for the Joint Domain Localized Generalized Likelihood Ratio Detector (JDL-GLR).
Another class of techniques, Reduced Rank STAP algorithms, use adaptive processing to exploit the fact that inherent dimensionality or rank of clutter or may be significantly lower than the rank of the covariance matrix. Reduced Rank STAP algorithms include Principal Components, Eigenfilters and the Cross-Spectral Metric. Reduced Rank STAP algorithms also include Multi-stage Wiener Filters (MWF) and Parametric Matched Filters (PMF) that do not explicitly require knowledge or estimation of the covariance. Reduced Rank STAP algorithms can also be combined with Post-Doppler and Beamspace techniques.
Diagonal loading and covariance matrix tapers can be employed to compensate for internal clutter motion and clutter non-stationarity.
The inclusion of fast-time DOF's and a technique referred to as 3-D STAP has been developed to suppress multipath scattering by a jammer. The jammer multipath is commonly referred to as “hot clutter”.
Performance may also be enhanced by the inclusion of elevation DOF's in addition to azimuth DOF's.
Analyses, simulations and experiments have demonstrated that the performance achieved with partially adaptive STAP can approach that of a fully adaptive STAP processor.
Performance of STAP algorithms may be described by a plot of the SINR Loss over the space-time aperture. For an ideal monostatic radar, the SINR Loss will be characterized by a sharp linear null that is aligned with the clutter locus. The width of the null is a measure of MDV. The width of the null depends directly on the selection of radar DOF's, the STAP algorithm and signal processing parameters.
Differences in STAP algorithms are the result of the need to compensate for the more complex signal environment in a practical system that may include mismatch in receiver channels, roll, yaw and pitch of the radar platform, jammers, clutter motion and clutter inhomogeneities.
In bistatic radar systems, the vectors from the transmitter to a reference point, from the receiver to the same reference point and from the receiver to the transmitter define the sides of a triangle. The included angle at the reference point is called the bistatic angle. The bistatic angle measures the departure of a bistatic sensor from monostatic operation. For monostatic radar the bistatic angle is zero.
Bistatic radar operation may enable improved detection performance. Unlike the transmitter, the receiver does not have a large electromagnetic signature and so can be located closer to targets of interest. The reduction in range translates into increased signal-to-noise ratio (SNR) and the potential to detect small targets that might fall below the noise level for a radar receiver co-located with the illumination source (i.e., for monostatic operation).
Bistatic radar receiver operation may enable improved performance in the presence of countermeasures. Because bistatic receivers are passive, electronic countermeasure systems designed to determine the location of a receiver and direct jammer energy toward it are reduced in effectiveness. Similarly, radar cross-section reduction technology designed to re-direct scattered energy away from the transmitter are reduced in effectiveness.
Similarly, passive operation at reduced ranges may enable covert and clandestine deployment throughout an area of interest.
Multiple bistatic radar receivers can simultaneously exploit a single radar transmitter. This mode of operation is one example of multistatic operation and can be used to mitigate the effect of radar cross-section fluctuations and/or multipath fading and improves performance compared to monostatic radar.
The linearity of the clutter locus is retained in certain bistatic radar systems, those for which the transmitter is stationary. In such a system, there is no Doppler on the transmitter to clutter propagation path and the Doppler on the clutter-to-receiver propagation is just ½ of that for a monostatic radar system. In addition, the slope of the clutter locus is range independent. Because of the range independence, characteristics of clutter in a given range cell, i.e., R, the space-time covariance of clutter measurements, can be estimated based on data in neighboring range cells. The clutter suppression filter can then be derived in terms of the estimated clutter covariance.
FIG. 2 shows 4 delay strips for a typical airborne bistatic scenario and demonstrates that very basic relationships are simply lost in a bistatic system when the transmitter and receiver motion are unconstrained. For the scenario shown in FIG. 2, the transmitter is heading Northeast with a speed of 180 km and the receiver is approximately 140 km Southeast of the transmitter. Lost are linear and invariant relationships between space-time sensor measurements, specifically, the linear and delay invariant relationship between Doppler and sine of the azimuth angle, relationships that are the basis for clutter and interference suppression technology in monostatic systems.
Current bistatic and multistatic radar designs design and derive DOF's that are similar to those in a monostatic radar. Current approaches to bistatics and multistatics, for example, view the transmitter as an independent and essentially non-adaptive component of the larger system. Spatial degrees of freedom (DOF's) are allocated, in their entirety, to the receiver. While adaptive signal processing including STAP is commonly employed, the signal processing does not include transmitter feedback or adaptation of the transmitter aperture.
Because bistatic and multistatic clutter is highly non-linear and non-stationary, the direct application of STAP techniques developed for monostatic radar will have limited capability to suppress or cancel clutter while maintaining moving target detectability.
Approaches to bistatic and multistatic clutter suppression fall into two general categories: 1) clutter tuning and 2) measurement compensation.
Clutter tuning refers to attempts to eliminate or minimize the need for clutter suppression and STAP processing. Clutter tuning effectively restricts platform trajectories or restricts segment of platform trajectories for which data is processed in an effort to minimize the region of Doppler measurements containing clutter. For an ideal clutter tuning scenario, the transmitter and receiver platforms are at the same velocity and are moving toward a common point. In this case, the Doppler of clutter is exactly zero and the effects of clutter are highly localized in measurement space. The localization of clutter in measurement space is approximately obtained for transmitter and receiver platform motion that departs from the conditions for ideal clutter tuning. A practical system based on clutter tuning requires, in general, an increased number of platforms to ensure detection of all targets.
Measurement compensation techniques typically invoke physical models to define groups of similar scattering centers in neighboring delay to predict, for these scatterers, the dependence of the clutter locus on delay, Doppler and/or angle. The delay, Doppler and angle measurements of similar scattering centers define a map between neighboring measurement bins. This map can be used to identify and compensate measurements that are used to estimate the covariance of clutter or to otherwise characterize the statistical properties for techniques such as the Multistage Wiener Filters that do not require explicit computation of the covariance, define a map between delay bins that identifies and to compensate for variation in Doppler and angle measurements.
Measurement compensation techniques include derivative based updating (DBU), Doppler compensation and Doppler-angle compensation. Measurement compensation are designed for integration with STAP techniques developed for monostatic radar.
A single preferred combination of measurement compensation and STAP techniques has not been established. The special case of a bistatic radar with a stationary transmitter illustrates certain issues. The Characteristic Clutter Locus for a bistatic radar with a stationary transmitter shares the same linear and delay independent characteristics of the monostatic radar as shown in FIG. 1. However, when the strength of the radar return is also taken into account, it is known that the peak of the return will migrate along the linear clutter locus as the delay bin is varied. It has been shown even though the clutter locus is linear and delay invariant, the performance of JDL-GLR STAP algorithms will be significantly degraded unless the STAP algorithm is integrated with angle-Doppler compensation. In contrast, the it has been shown that Parametric Match Filters (PMF) can achieve effective clutter suppression without the integration of angle-Doppler compensation.